This invention concerns beam splitters and in particular those which produce a plurality of parallel beams from a single input beam. These beams are sometimes used to image on a surface an array of dots whose individual intensities can be controlled, for example in laser half-tone printing.
In laser printing applications, the output beams are generally required to be parallel, of equal intensity and equally spaced from each other. One apparatus for producing any desired number of beams from a single input beam comprises a number of half-silvered mirrors, that number being one less than the number of output beams required. Each half-silvered mirror receives only one input beam and splits that beam into mutually perpendicular transmitted and reflected beams of equal intensity. The half-silvered mirrors are arranged in parallel and in series so that each of the final output beams has undergone the same number of reflections and/or transmissions. Further mirrors are also required to bring the output beams into a parallel, equally-spaced relationship. This apparatus is unsatisfactory due to the large number of mirrors required and the difficulty of arranging their relative positions.
An improved beam splitter is described in our British patent No. 1522555. This beam splitter consists of just one block with parallel faces for receiving one input beam and for producing several output beams. Its parallel faces are coated in a manner which varies along the length of the block so as to vary the reflection/transmission coefficients along its length ensuring that at successive internal reflections the transmitted beams emerge parallel and of equal intensity. This beam splitter, however, suffers from the major disadvantage of cost of manufacture consequent on the need for the opposite faces to be accurately parallel and to receive many coating operations. The effect of the out-of-parallelism is progressive on successive transmitted beams. The opposite faces require a number of different coatings equal to the number of emitted beams, each coating applied under a vacuum, and, once constructed, the device cannot vary the spacing of the output beams, and the beams can never be contiguous. Moreover it is not feasible to multiply the number of beams produced, simply by combining like beam splitters in cascade. This is because successive beam splitters would require more closely spaced coatings, so that it would be simpler to derive all the beams from the first splitter by making it longer.